Strontium-Doped Hydroxyapatite as a Multifunctional Platform: Tailoring Structural, Mechanical, and Drug Delivery Properties for Bone-Related Applications

Abstract

Hydroxyapatite (HAp) is a calcium phosphate ceramic that has been extensively studied for its similarity to the mineral phase of bone tissue, and it exhibits excellent biocompatibility and osteoconductivity. However, its intrinsic brittleness and low fracture resistance limit its use in load-bearing environments, motivating the use of ionic doping to enhance its properties. Doping with strontium (Sr) is particularly advantageous, as this ion is associated with bone regeneration processes, improves osteogenic response, and favorably modifies the material’s lattice parameters and structural organization. In this work, Sr-doped HAp nanorods (5, 10, and 20%) were synthesized by the hydrothermal method, and their structural, mechanical, and drug delivery properties were investigated, with mechanical testing performed on non-sintered pellets to preserve the characteristics of the as-synthesized material. Drug incorporation and release assays with ciprofloxacin were conducted to evaluate the multifunctional potential for bone healing applications. The results confirmed the successful incorporation of Sr2+ into the hydroxyapatite lattice without the formation of secondary phases. Sr incorporation promoted lattice expansion, resulting in a maximum unit cell volume increase of 10 Å3, reduced nanorod length, and increased diffraction peak broadening, while Rietveld refinement revealed Sr occupancies of up to 19.5%. Furthermore, Sr incorporation increased the fracture toughness of the compacted non-sintered materials by up to 385% compared with pure HAp, while preserving the hydroxyapatite structure and enabling efficient ciprofloxacin loading and sustained release. Overall, Sr-doped HAp nanorods demonstrate tunable mechanical and functional properties, highlighting their potential as multifunctional materials for bone-related applications requiring both structural performance and localized therapeutic action.